A spectacle blind is a safety device used to isolate a section of line or piece of equipment when the line or equipment needs to be inspected or removed from service. It is different than a valve in that the blind is a permanent or long term isolation device.
A spectacle blind is machined from a single piece of metal that is cut to match the pipe size, fit between two pipe flanges and requires an additional gasket when it is installed. Also, the bolts will need to be lengthened depending on what piping class and size blind is used. The thickness of the spectacle blind is specified based on the line pressure and pipe size.
The specification that determines the dimensions of a spectacle blinds is ASME B16.48 - Line Blanks. A line blank conforming to this standard will be marked in the following way:
- Nominal Pipe Size (NPS)
- Nominal Pressure Class
- Manufacturer's Name
- If RTJ, the ring number
One end of the blind will have an opening to allow flow through the pipe during operation and the other end is solid to block flow during maintenance.
Ring spacers are bored to the matching pipe ID and are the same thickness as the "single blind" that it replaces. When removing a "single blind", either the flange and associated piping must be pulled together to seal the line, or a "ring spacer" must be installed to fill the gap. Thick single blinds or rigid piping systems normally require ring spacers.
Single/Line Blind or Blank
A positive shut-off device normally installed adjacent to, or in conjunction with, a valve. Their purpose is to prevent accidental flow through a pipeline to a vessel. With the exception of cast iron, plastic, or fiberglass services, they are not drilled with bolt holes, but fit inside the bolt circle of mating flanges. Pipeline blinds or blanks are not the same as bolting blind flanges. Single blinds ues standard gaskets.
A combination of a "single blind' and a "ring spacer" can be fabricated for field convenience as a single unit. Weight consideration and the associated difficulty of handling heavy pieces in the field are a primary consideration in specifying a "spectacle blind" or a combination of blinds. Spectacle blinds are meant to be rotated to change blind/spacer orientation.
A spec blind is a combination of a ring spacer and single blind. They are usually permanently installed in a piping system and rotated as needed.
Similar to a "single blind", but thinner, normally 1/8" (3mm) to 5/16" (8mm) thick. These are positive sealing devices intended to prevent accidental flow or leakage of vapors into a pipeline or vessel, usually while the system is in service. Vapor blinds are not to be subject to differential pressure.
A test blank is specially designed blank used for hydrostatic or other incompressible fluid testing purposes only. Their advantage is cost and weight savings since higher allowable stress values (or lower safety factors) are used in their design.